Weak, Seismogenic Faults in the Lower Crust

Earthquake-generating faults are typically confined to the upper 10-15 km of the crust, with the middle and lower crust deforming aseismically. Along the margins of Earth’s highest mountain ranges, however, seismicity can extend throughout the whole crust, from the surface to depths of 30-50 km in rocks at temperatures of 400-600 degrees. For earthquakes to take place at such high temperatures, the lower crust is thought to have an extremely dry (anhydrous) mineralogy, such that elastic strain is not relaxed by creep.

In this study, I will discuss the mechanical properties of earthquake-generating faults in the lower crust around the Andes mountains. I will use force-balance calculations to demonstrate that faults within the lower crust can be frictionally very weak, with an average effective static coefficient of friction <0.2. The mechanisms invoked to generate similar frictionally-weak, earthquake-generating faults in the upper crust appeal to the presence of highly-pressurised water, or water-driven alteration of the fault core to form phyllosilicate minerals. However, the dry mineralogy thought to necessitate elastic strain accumulation in the lower crust should preclude abundant free water within these faults by acting as a `sponge’, soaking up free water in hydration reactions. The geological controls on the frictional properties of earthquake-generating, lower-crustal faults remain a conundrum.